If the TV was built after mid 2007, and has any tuner built in, it should already be fully capable of receiving over the air digital TV signals. This is an FCC mandate that applies to TV's sold in the US.
Most TV tuners default to 'cable' mode which uses a different digital format than OTA digital broadcasts. If you currently have cable service, you'll need to get out the user manual and follow the manufacturer's instructions to re-configure the tuner from 'cable' mode to 'air' or 'antenna'. Typically, once that's done and you have an antenna connected, you'll then need to run a new scan for channels.
If you need help selecting or installing an antenna, please post a link to your TV Fool reception report.
Having said all that, Channel Master offers several options. The CM-7001 is a tuner capable of receiving 1080i, 720p and 480i OTA content. They also offer a couple of tuner/DVR options in the CM-7400 and CM-7000PAL.
http://www.channelmaster.com/Set_Top...e_DVR_s/40.htm
There are other products as well,
http://forum.tvfool.com/showthread.php?t=2882.